Amplifier with controlled gain

摘要

Amplifier gain is varied in response to a control voltage by a switched capacitor variable transconductance feedback network. First and second capacitors are connected in series between the input and the output of an operational amplifier. The input side of the first capacitor is switched by a first toggle switch between the input and ground. The output side of the second capacitor is switched by a second toggle switch between the output side and ground. Both toggle switches are operated by a pair of non-overlapping pulse trains. A voltage-controlled variable resistance network is connected between the common node of the capacitors and ground. It includes a first enhancement mode NMOS transistor having its source connected to the common node and its gate and drain connected to ground, together with an enhancement mode NMOS bias current transistor having its drain connected to the source of the first transistor, its source connected to a negative voltage supply, and its gate connected to a control voltage source. Also disclosed are distortion suppression networks which can be added to the above circuit to reduce distortion at high input signal levels and an alternate variable resistance network controlled by the same control voltage and containing a resistance branch between the common node of the first and second capacitors and the input of the operational amplifier.